1. Field of the Invention
The present invention relates to a solid state imaging device, and more particularly, to a solid state imaging device having its improved sensitivity.
2. Description of the Related Art
As an imaging device, a so-called complementary metal oxide semiconductor (CMOS) sensor is known. The CMOS sensor has unit cells arranged therein. Each of the unit cells is composed of a photoelectric converter (photodiode), a readout transistor, a reset transistor, an amplifier transistor, and a selector transistor. The readout transistor reads out a signal from the photoelectric converter. The reset transistor resets a signal electric charge. The amplifier transistor amplifies a signal. The selector transistor selects a line.
In order to increase the sensitivity of the photodiode, Jpn. Pat. Appln. KOKAI Publication No. 2003-282857 has proposed a structure in which the photodiode extends under a detector section of one's own pixel. One pixel is defined as a region which a light beam enters and in which the photodiode carries out photoelectric conversion and a transistor for processing the signal electric charge is formed.
In this case, the photodiode is formed under the readout transistor. Thus, in order to stably make an operation of the readout transistor, in general, it is necessary to form a semiconductor layer whose conductivity type is opposite to the photodiode under the readout transistor. This is because, if such a semiconductor layer (p-type) whose conductivity type is opposite to the photodiode (n-type semiconductor layer in this case) is not formed under the readout transistor, the photodiode and the detector section (conductive semiconductor layer having the same conductivity type as the photodiode: n-type in this case) may be electrically connected to each other.
However, if the semiconductor layer with a conductivity type (p-type) opposite to the photodiode is formed under the readout transistor, a need for adjusting a threshold value of the readout transistor arises. This is one of the factors of narrowing an operating margin of the readout transistor and unstable device operation.